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We consider a novel graph-based problem, in which a population of arbitrary size aims at achieving a common objective. More specifically, WinPop is a synthesis problem defined by a finite graph with edges labels in {✓, -, x}. The instance is positive if there exists a sequence (π_i)_{i ∈ ℕ} of infinite paths such that for any fixed population size N ∈ ℕ_{> 0}, there is a path whose N-th transition is labelled ✓ and all previous paths have their N-th transition labelled by -. Alternatively, WinPop can also be cast as a 2D-tiling problem with vertical and horizontal constraints: the horizontal constraint reflects the possible paths in the input graph, and the vertical one encodes that a ✓-label eventually occurs, before any x-label. Finally, WinPop also corresponds to the existence of a coalition strategy for a reachability objective in parameterized concurrent games. We use algebraic tools to show that the problem can be solved in polynomial space. First we exhibit a finite semigroup whose elements summarize coalition strategies over a finite interval of population sizes. Then, we characterize the existence of winning strategies by the existence of particular elements in this semigroup. Finally, we provide a matching complexity lower bound, to conclude that WinPop is PSPACE-complete.